1. Field of the Invention
The present invention relates generally to organic compositions containing a metal, and more specifically, to polymer compositions containing metallocene(s) and/or metal nanoparticles and carbon compositions containing metal nanoparticles homogeneously dispersed therethrough, and methods for manufacturing.
2. Description of the Background Art
Because of processing advantages, polymers containing transition metals can be of considerable interest as potential pyrolytic precursors to metallic nanoparticle carbon and ceramic compositions. The design and synthesis of macromolecules that incorporate transition metals or transition metal complexes as essential structural elements constitutes an area of growing research interest promoted largely by the prospect that such materials may possess novel catalytic, optical, redox, magnetic, or electrical properties. The use of such materials as precursors to transition metal-containing carbon and ceramic compositions is virtually unexplored.
Most approaches to such materials have involved chemical modification of preformed preceramic polymers such as the derivatization of liquid poly(methylsilanes) by dehydrogenative coupling using MMe2(C5H5)2 (M=Ti, Zr, or Hf) which yields SiC/MC composites after pyrolysis. Studies of the thermal stability by thermogravimetric analysis (TGA) have shown that these polymers undergo significant weight loss (35-56%) between 350-500° C. but show no further weight loss up to 1000° C. The lustrous ceramic products formed when ferrocene-containing polymers are heated to about 500° C. under a slow flow of nitrogen are ferromagnetic and have been characterized as amorphous iron silicon carbide materials by X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDA) microanalysis. Other approaches have involved the pyrolysis of poly(silylacetylenes) or other silicon-containing polymers in the presence of transition-metal powders or oxides to yield MC/MSi/SiC or SiC/MN composites.
Carbon-carbon (C—C) composites are emerging materials with potential applications as high temperature structural materials for the space and aerospace industries. This is because they possess not only light-weight but high thermal conductivity and high dimensional stability up to 3000° C. in a protective environment. Carbon-carbon composites are prepared by using carbon fibers as reinforcement and thermosetting or thermoplastic resins as matrix precursors. On pyrolysis, the thermosetting resin yields a non-graphitic matrix whereas the thermoplastic resin leads to graphitic carbon. Matrix microstructure plays an important role in deciding the overall performance and particularly the mechanical properties of C—C composites. Numerous studies have been carried out to understand and control the microstructure and the degree of graphitization in the developing matrix under various carbonization conditions. In addition to carbon precursor material, the heat treatment temperature, which is typically between 2000 and 3000° C., also influences the degree of graphitization. Several methods have been investigated to improve the degree of graphitization under milder conditions. One such method is catalytic graphitization, which is brought about by addition of certain inorganic and organic additives to the matrix precursors before conversion to the carbonaceous matrix. The overall effect is to lower the temperature needed to achieve a certain degree of graphitization. Very little research has been reported using melt processable polymeric and carbon precursor materials containing organometallic groups. During the heat treatment, the organometallic units are destroyed and the metal particles are molecularly dispersed homogeneously throughout the matrix.
Dispersion of very small metal particles in polymeric, carbon, or ceramic matrices are scientifically and technologically important for a variety of reasons. The preparation of nanoscale materials with unique properties represents a significant challenge. Materials with particles in the range of 1-10 nanometers would be expected to exhibit unique properties due to their extremely small dimensions. One potential advantage of a dispersed particle system is that many of its properties are strongly dependent on the interfacial properties of the materials because the fraction of the overall materials, which is in the vicinity of the fraction of an interface, is quite high. In addition to simply providing a large interfacial area, dispersions of very small inorganic particles may have useful electronic, optical, magnetic, chemical, catalytic and unique mechanical properties. Many of these properties such as superparamagnetism in magnetic materials or tunable band gaps in quantum dot semiconductor arrays require that particle sizes and interparticle spacings should be in the nanometer range. Because of the similarity of these dimensions to the typical sizes of polymeric molecules, there is considerable interest in using polymeric materials to control the sizes and distributions of nanoparticle dispersions. One approach that has been employed by several groups is to use ordered block copolymers as templates for controlling the distributions of the metal particles. Unfortunately, the morphology of the underlying polymeric template and the morphology of the resultant particle dispersion were not related to each other in a completely straightforward way to afford uniform particle dispersion. This result can be partially explained in terms of kinetic limitations on particle coalescence in the presence of the polymer molecules in the solid state. Moreover, well-ordered arrays of metal particles with controlled sizes have been obtained in solvent-cast films. For practical reasons, it would be convenient to process and control metal dispersions from decomposition of organometallic containing polymers or compounds in the melt state. For example, the viscosity of the composition would be expected to control the diffusive properties of the individual metal particles and particle coalescence.
Molecular or macromolecular materials, which contain atoms or transition metal elements in close proximity, are attracting increasing attention because of their potentially interesting electrical, redox, optical, and magnetic characteristics. Small particles with diameters less than 10 nanometers exhibit material properties that strongly differ from those of bulk phases. Their electronic, magnetic, and optical properties contribute attractive prospects in the design of new electronic and optical devices, information storage, color imaging, bioprocessing, magnetic refrigeration, ferrofluids, gas sensors, etc. In these applications, small particles of metal oxides have been shown to be particularly important.
The general properties of metal nanoparticles or metal clusters of one to a few nanometers can be of immense importance on the basis of numerous investigations in the last few years. The physical properties of metal nanoparticles are attracting increasing interest because they may differ significantly from the bulk properties as a result of surface or quantum size effects. Quantum size effects open the door to novel future technologies. The success of such future applications of metal nanoparticles will strongly depend on the availability of one, two, or three dimensionally organized materials. To study quantum size behavior of small metal particles, the particles must be separated from each other to avoid coalescence and to keep the individual nature of the particles. Small crystallites behave like large molecules (e.g., polycyclic aromatic hydrocarbons) in their spectroscopic properties. They do not exhibit bulk semiconductor electronic properties. Metal nanoparticles or nanoclusters can be considered the building blocks of future modern technologies. This will be due to the size dependent electronic properties of these particles. Nanoparticles of transition metals become semiconductors if small enough. As the more technological problems such as organization and addressing of quantum dots are better understood, there is an almost unlimited field of applications to be foreseen. The properties of nanosized semiconductors have long been known to depend very sensitively on the particle size.